This invention relates to a rare earth element-Fe-B permanent magnet powder and a rare earth element-Fe-Co-B permanent magnet powder, which are excellent in magnetic anisotropy and corrosion resistivity, and bonded magnets manufactured therefrom.
Rare earth element-Fe-B alloys which are mainly composed of at least one of rare earth elements including yttrium (hereinafter referred to as "R"), Fe, and B recently drew attention as materials for permanent magnet powders excellent in magnetic properties and have widely been developed mainly for use as magnet powders for bonded magnets.
In general, bonded magnets have higher physical strength than sintered magnets though they have lower magnetic properties as compared with sintered magnets formed of the same kind of magnetic materials. Further, bonded magnets also have higher formability into various shapes than sintered magnets. For these reasons, they have been finding an increasing range of applications. A bonded magnet is usually formed of a magnet powder, and an organic binder or a metallic binder which are bonded together, and has magnetic properties dependent on magnetic properties of the magnet powder forming the magnet.
One of R-Fe-B permanent magnet powders for use in manufacturing bonded magnets mentioned above has been proposed by Japanese Provisional Patent Publication (Kokai) No. 1-132106. The proposed R-Fe-B permanent magnet powder is formed of a R-Fe-B master alloy as a raw material, the alloy having a main phase formed by a R.sub.2 Fe.sub.14 B type intermetallic compound phase which is a ferromagnetic phase (hereinafter referred to as "R.sub.2 Fe.sub.14 B type phase). The R-Fe-B permanent magnet powder is manufactured by subjecting the master alloy material to a heat treatment in a H.sub.2 atmosphere at a temperature within a predetermined range to be transformed in phase into respective phases of RH.sub.x, Fe.sub.2 B and the balance of Fe, and then subjecting the phase-transformed alloy material to a dehydrogenation treatment to have H.sub.2 removed therefrom to form a R.sub.2 Fe.sub.14 B type phase which is a ferrormagnetic phase, again. The resulting R-Fe-B permanent magnet powder has an aggregated structure having a main phase formed by a very fine R.sub.2 Fe.sub.14 B type recrystallized structure with an average grain size of 0.05-3 .mu.m.
A R-Fe-Co-B permanent magnet powder is also described in Japanese Provisional Patent Publication (Kokai) No. 1-132106, referred to hereinbefore. Also this R-Fe-Co-B permanent magnet powder has an aggregated structure having a main phase formed by a very fine R.sub.2 (Fe, Co).sub.14 B type recrystallized structure with an average grain size of 0.05-3 .mu.m, wherein part of the Fe is replaced by Co.
The conventional R-Fe-B permanent magnet powder and R-Fe-Co-B permanent magnet powder have the following disadvantages:
(i) Although they have some magnetic anisotropy, this magnetic anisotropy can be degraded depending on slight variations in the alloy composition and/or the manufacturing conditions, which makes it difficult to obtain stable and excellent magnetic anisotropy;
(ii) To impart magnetic anisotropy to a R-Fe-B or R-Fe-Co-B permanent magnet powder, it is generally known to subject such a magnet powder to hot deformation processing such as hot rolling and hot extrusion to flatten the crystalline grains. This known method can enhance the magnetic anisotropy to some degree. However, the grain-flattened permanent magnet powder inevitably has local variations in the reduction ratio. Thus, the known method is not only unable to obtain a R-Fe-B or R-Fe-Co-B permanent magnet powder which has a stable homogeneous magnetic anisotropy, but also requires complicated manufacturing steps and hence causes a high manufacturing cost.
(iii) A R-Fe-B or R-Fe-Co-B permanent magnet powder having recrystallized grains flattened by hot deformation processing is more susceptible to corrosion than a R-F-B or R-Fe-Co-B permanent magnet powder having non-flattened recrystallized grains. If such a R-Fe-B or R-Fe-Co-B permanent magnet powder with flattened recrystallized grains is stored under a hot and humid atmosphere such as in the manufacturing plant for a long time, its surface will corrode, resulting in degraded magnetic properties.